1. Field of the Invention
The present invention relates to a technique for acquiring spectral radiance factors of fluorescence.
2. Description of the Related Art
A material that produces fluorescence (for example, a fluorescent substance such as a fluorescent whitener) is often used in a print medium and color materials used in an image output operation, so as to enhance the whiteness of the print medium and to revive colors of the color materials. The fluorescent substance has a property of emitting light within a visible light range when it is exposed with light outside the visible light range represented by ultraviolet rays. As a result, the color reproducibility of a printed matter varies depending on differences of illumination light used to view an output image. Therefore, in an image processing apparatus which uses a print medium and color materials containing a fluorescent substance, in order to faithfully estimate reproduced colors of an output image under an arbitrary viewing environment, it is required to acquire spectral radiance factors due to light emission of the fluorescent substance contained in the print medium and color materials.
In general, spectral reflectances of a sample are measured by exposing the sample with light coming from a white light source, receiving reflected light by a spectroradiometer to measure spectral reflection intensities, and dividing the spectral reflection intensities by a spectral intensity of the white light source. However, with this measurement method, the spectral reflection intensities of the sample cannot be separated into reflection elements due to regularly reflected light and diffuse light, and fluorescent elements due to emission (fluorescence) of the fluorescent substance. Therefore, fluorescent-dependent spectral reflectances of the measured spectral reflectances, that is, spectral radiance factors of fluorescence alone cannot be acquired.
Therefore, in order to measure fluorescence emitted by a sample, the following technique has been proposed. A method of measuring spectral reflectances of a sample under two different light sources, and estimating a fluorescent amount under a viewing light source based on a ratio between a light energy sum total in an excitation spectrum range and a light energy sum total in an emission spectrum range of the respective light sources is known (for example, Japanese Patent Laid-Open No. 2006-84333; to be referred to as literature 1 hereinafter). Also, a technique which configures a colorimeter that measures fluorescence alone is known (for example, Koshin Mihasi, “Fluorescent measurement”, The Spectroscopical Society of Japan, Measuring Method Series, Japan Scientific Societies Press, 1983; to be referred to as literature 2 hereinafter). According to this technique, arbitrary short-wavelength light in a spectrum range outside the visible light range is extracted from light coming from a measuring light source via a spectroscope as incident light. Then, fluorescence yielded by exposing a sample with the incident light is input to a detector via the spectroscope again.
However, the conventional fluorescent measuring method suffers the following problems.
With the technique described in literature 1, since the fluorescent amount of the sample under the viewing environment is calculated from measurement values under the two light sources, the two different light sources have to be prepared, thus complicating the arrangement of a colorimeter.
A fluorescent measuring apparatus described in literature 2 requires a spectroscope to generate incident light onto the sample, thus also complicating the arrangement of the colorimeter. Furthermore, since a spectral radiant energy of short-wavelength light acquired from the light of the measuring light source via the spectroscope lowers, a spectral reflectance energy of fluorescence emitted by the sample with respect to the incident light also lowers. For this reason, it is difficult to detect fluorescence by the detector, resulting in poor measurement precision. Hence, the detection precision of the detector has to be improved to be higher than a general spectroradiometer, or an amplifier which enhances the lowered spectral reflectance energy of fluorescence is also required. Therefore, the arrangement of the fluorescent measuring apparatus based on the above technique is further complicated as a specific apparatus compared to a general colorimeter. Also, in order to acquire fluorescence from the sample, light outside the visible light range has to be exposed. With the technique described in patent literature 1, in order to extract the incident light as short-wavelength light, a plurality of different short-wavelength light rays have to be incident in a range outside the visible light range, and have to be measured a plurality of times, thus requiring a larger number of measurement processes.